1. Field of the Invention
The present invention relates to a semiconductor laser of terraced substrate type.
2. Description of the Prior Art
Semiconductor laser has advantages of smallness in bulk, high efficiency and direct modulation by means of its current. As a result of drastical development of the semiconductor lasers in recent years, the lasers have become widely considered to be used as light sources for various kinds of apparatus such as for light communication, video-disk recording and prproducing, audio disk playing laser printing, hologram, and etc. For most of these uses, it is strongly requested that the oscillation of the semiconductor laser being a single transverse mode.
The conventional stripe type laser has a structure of lasing region of simple gain guiding, which does not have a particular structure to confine light in one part of the active layer, but the oscillation is likely to be carried out in the whole width of the active layer. Therefore, the conventional laser has had a difficulty in maintaining a transverse mode for a wide range of current, and therefore has been liable to occurrence of undesirable mode conversion or a generation of higher modes. As a result of these, the light-current characteristic of the conventional laser has been likely to have a kink of characteristic curve or the device has been likely to have a multiple longitudinal mode oscillation.
In order to solve such problem, there has been proposed special type semiconductor laser structure named terrace-substrate type (TS type) laser, and many improvements have been proposed. Among the proposed improvements of the TS type lasers, such improved structure as shown by FIG. 1 has been proposed in the Japanese Patent Application No. Sho 55-13159 and corresponding U.S. Pat. Ser. No. 224,821, Canadian Patent Application No. 368,427 and European Patent Application No. 81100192.4, neither of them is disclosed or published yet. The laser of the abovementioned proposed invention has a double-hetero structure, as shown in FIG. 1, which has on
a terraced substrate 1 of . . . n-GaAs PA1 a first clad layer 2 of . . . n-GaAlAs, PA1 an active layer 3 of . . . non-doped GaAlAs, PA1 a second clad layer 4 of . . . p-GaAlAs and PA1 a cap layer 5 of . . . n-GaAs, wherethrough
a Zn diffused p-type current injection region 6 is formed in a manner that its diffusion front slightly goes down into the underlying second clad layer 4. A p-side electrode 7 is formed on the cap layer 5 forming the ohmic contact to the current injection region 6. The cap layer 5 of n-GaAs and the underlying second clad layer 4 of p-GaAlAs form an isolation junction inbetween except the part of the current injection region 6.
Such type of the TS type laser can effectively confine laser oscillation light in the oblique active region of the GaAlAs active layer 3 formed at the vicinity of the step part of the terraced substrate 1, and furthermore the current injection region is also effectively limited by the width of contact between the diffused region 6 and the second clad layer 4. Therefore, the abovementioned laser can stably oscillate at the single mode and low threshold current.
Hereupon, in order to effectively radiate the heat produced at the active layer 3, generally the semiconductor lasers are bonded on a heat sink by the top side electrode 7, which is closer to the active layer 3, not by the bottom side electrode 8, which is separated from the active layer 3 by about 100 .mu.m thick substrate 1. That is, the semiconductor laser is generally upside-down bonded on a heat sink by known low melting point solder such as indium metal. In such upside-down bonding, the TS type semiconductor laser has a problem that its top face, that is, the p-side electrode 7 in the example of FIG. 1 has a slightly terraced surface, since the substrate is of terraced shape. Accordingly, in the bonding process of the TS laser in the upside-down manner on a heat sink with the solder layer inbetween, because of non-flatness of the top face of the laser chip the solder is likely to be pushed out from the periphery of the semiconductor laser chip when pressing the semiconductor laser chip on the heat sink. And this pushed out solder is likely to creep up to the lower part of the peripheral section or side face of the semiconductor laser chip. Such creeping up on the side face is likely to cause short-circuiting of the p-n junction between the layers 3 and 4 of the laser chip, since the cap layer 5 in the thinner part (left half part in FIG. 1) is only about 1 .mu.m. Actually, such short-circuiting very often has occurred in manufacturing the TS type laser than other type. In order to eliminate such problem, to increase thickness of the cap layer 5 has been examined thereby intending to obtain more flat upper face. But the increased thickness on n-GaAs layer 5 makes an accurate control of the diffusion depth of the Zn-diffused region 6 difficult, that is the control of the effective width of the current injection path becomes difficult.